This invention relates generally to a reduction gear assembly for an electronic timepiece in which the minute and hour hands may be manually set without rotating the seconds hand. More particularly, the invention relates to an improved friction drive/slip coupling in a wheel and pinion reduction gear assembly for use in a timepiece.
Timepieces for many years have employed gear trains incorporating a friction drive/slip coupling at some point in the gear train. The purpose of such a friction drive/slip coupling is to allow one section of the gear train to be rotated, during setting the hour and minute hands of timepiece, while the other section of the gear train remains locked to prevent rotation of the seconds hand. An example of a friction drive with slippage for setting the timepiece is seen in U.S. Pat. No. 3,290,875 - Egger, et al, assigned to applicant's assignee, wherein a spring plate or washer provides a friction drive between a pinion member and a wheel member, which together make up a reduction gear for the so-called "third wheel assembly".
Another example of a friction coupling in a wheel and pinion reduction gear is seen in U.S. Pat. No. 4,932,011, also assigned to applicant's assignee, wherein a friction drive/slip coupling is formed between a pinion and wheel by inserting a slotted end of the wheel shaft into a hub on the pinion.
A similar friction coupling between two gear members is also seen in Swiss Patent 5739 issued 10 Oct. 1892 to Lugrin.
One of the problems with friction couplings of the type described is the inability to obtain consistent values of the breakaway torque required to slip the coupling. The systems disclosed in the prior art mentioned above are both expensive to manufacture and do not always give consistent torque readings. Particularly when attempting to reap the benefits of lower cost plastic components, known types of slip couplings employing plastic-to-plastic friction surfaces are quite unreliable.
Accordingly, one object of the present invention is to provide an improved reduction gear assembly for a wheel and pinion with an improved friction drive/slip coupling between the pinion and wheel members.
Another object of the invention is to provide an improved and lower cost reduction gear assembly for the "third wheel" of a timepiece.
Still another object of the invention is to provide an improved reduction gear assembly of a metal pinion and a plastic wheel providing a friction drive with a more reliable consistent range for the torque necessary to provide slippage between the members.